Eager Load Depending on Type of Association in Ruby on Rails

This is now working in Rails v6.0.0.rc1: https://github.com/rails/rails/pull/32655

You can do .includes(resource: [:address, :location])

You can do that with the help of ActiveRecord::Associations::Preloader class. Here is the code:

@issues = Issue.all # Or whatever query
ActiveRecord::Associations::Preloader.new.preload(@issues.select { |i| i.resource_type == "Order" }, { resource: :address })
ActiveRecord::Associations::Preloader.new.preload(@issues.select { |i| i.resource_type == "Customer" }, { resource: :location })

You can use different approach when filtering the collection. For example, in my project I am using group_by

groups = sale_items.group_by(&:item_type)
groups.each do |type, items|
  conditions = case type
  when "Product" then :item
  when "Service" then { item: { service: [:group] } }
end

ActiveRecord::Associations::Preloader.new.preload(items, conditions)

You can easily wrap this code in some helper class and use it in different parts of your app.